The invention relates to a process for crosslinked branched polyesters, and their use as a chewing gum base and in non-food applications.
Aromatic polyesters such as poly (ethylene terephthalate) (PET), which are high melting and which have a high degree of cystallinity, are widely used in various molding and extrusion applications. Such applications include films, sheeting, bottles, containers and the like. Aliphatic polyesters generally have low melting points and are therefore much less useful in typical industrial applications. However, it was recently reported that certain aliphatic polyesters based on monomers approved for food applications are useful as a chewing gum base. It would therefore be beneficial to provide for improved polyesters for chewing gum bases and for non-food applications and improved methods of making such material.
The preparation of polyesters is described in a recent book by George Odian, Principles of polymerization, 2nd edition, pages 102-105 (1981), John Wiley and Sons, NY. Branched or crosslinked polymers can be prepared by using at least some monomers having a functionality of at least three. An apparatus for making such branched polymers is shown on page 132 of this same reference.
WO 98/17123 and WO 98/17124 patents to Wm. Wrigley Jr. Company describe a gum base including at least one aliphatic polyester that is produced from glycerol, propylene glycol or 1,3-butylene glycol and an aliphatic acid containing 4 to 12 carbon atoms.
The reactive extrusion process is a time-dependent process in which the condensation reaction conversion and viscosity are coupled. Due to the coupled nature of the system, the reactive extrusion process is less stable in short-barrel processors with an L/D less than 10, such as units manufactured by Readco Products of York, Pensylvania. Extruders with L/D greater than 25, such as Werner and Pfleiderer 30 mm unit with L/D=40, provide a more stable operation, but are not cost effective as the Readco continuous processors. The throughput achieved in the reactive extrusion is roughly 10 lb/hr in a 2xe2x80x3 Readco processor when a feed of roughly 0.70 conversion (absolute viscosity of 5 poise) to a gel greater than 0.77 conversion (absolute viscosity greater than 70,000 poise). However, this much conversion is not practical in a single extruder with L/D less than 25 due to process instability. One of the major drawbacks with the reactive extrusion process is that water removal is limited by mass transfer. Variability in the effectiveness of water removal in the extruder is believed to be a contributing factor to the inherent instability of the extrusion process in equipment with L/D less than 40 since the hold-up and residence time are directly related to viscosity and viscosity to the extent of reaction.
Other existing process technology to produce polyester resins by condensation polumerization typically utilizes vacuum process and/or inert gas stripping to facilitate the removal of water produced by polymerization. In particular, U.S. Pat. No. 5,714,553 to Kim, et al discloses a process performed in a continuously stirred tank reactor (xe2x80x9cCSTRxe2x80x9d).
U.S. Pat. No. 3,535,280 to Court, et al., teaches condensation polymerization of thermoplastics, including polycarbonates, in a wiped, thin film reactor.
A wiped, thin-film apparatus described in U.S. Pat. No. 3,695,327 to Widmer is designed specifically for vacuum processing materials of high viscosity. An aspect of that invention relates primarily to the blade and wiper designs used for transport of viscous materials as a thin film.
In U.S. Pat. No. 3,522,214 to Crawford et al. discloses a condensation reaction of viscous linear pre-polymers, including polyethylene terephthalate, using an extruder or other processor configured with screw flights and designed for high hold up, thin-film processing under vacuum
U.S. Pat. No. 4,237,261 to Kawamura et al. teaches that viscosity or conversion is controlled by adjusting the vacuum level in various stirred reactors, when the mixing power is used as a process indication of viscosity.
U.S. Pat. Nos. 4,319,017, 4,415,721, 4,465,819 and 4,465,819 to Kosanovich et al. disclose processes to produce thermotropic linear polyesters of aromatic dicarboxylic acids and diphenols. A two-step process is presented wherein a pre-polymer is prepared in the first step and subsequently reacted in a wiped-film reactor capable of high shear stress.
U.S. Pat. No. 5,302,255 to Dorai, et al. teaches that molecular weight distribution (polydispersity) of polyether glycols may be affected by short-path distillation. The use of two or more short-path stills in series is presented as a preferred process.
U.S. Pat. No. 4,474,938 to Richardson teaches a process for the polymerization of thermotropic linear aromatic polyester utilizing a wiped film reactor. The process in this patent is a two step process where a pre-polymer is prepared and then reacted using a single pass through the thin-film reactor.
U.S. Pat. No. 5,616,681 to Itoh et al. discloses a process for producing a linear biodegradable, aliphatic polyester at pressures between 0.3 and 3.0 Torr having high molecular weight.
DD 259,198 to Funk, et. al. teaches the polycondensation of polyethylene terephthalates. A novel reactor design is presented wherein a cylindrical, jacketed vessel with spray nozzles and baffles (fins) are provided to reduce entrainment.
None of the references disclose a process for making a crosslinked branched polyester pre-gel or gel that may be aliphatic and biodegradable for food applications, or aromatic or non-biodegradable for non-food applications. The disadvantages of the prior art are that the reactive extrusion process is relatively unstable due to the L/D limitations when more than 5% conversion is required in the extruder. Furthermore, the presence of unreacted adipic and palmitic acids, as well as the presence of low molecular weight oligomers affects the solubility of the final product in water. The processes of the prior art for crosslinked polyesters do not provide means for removal of substantially all of the unreacted acids and the low molecular weight oligomers as addressed by an embodiment of the present process of the invention. An additional drawback of the prior art reactive extrusion process is that it is both inefficient and costly to perform. Thus, there is a need to develop a stable commercially viable process for converting a pre-gel feed material to a crosslinked branched polyester gel in a controlled manner.
The present invention addresses the prior art problems by providing a low molecular weight pre-gel feed material to a continuous thin film reactive vacuum processing stage to obtain a high molecular weight pre-gel that is subsequently converted into a crosslinked gel by curing or by reactive extrusion.
Some advantages of the present process include:
(1) Branched cross-linked polyester gel polymers are produced from pre-gel using a stable process employing a batch reactor stage and a thin film reactive vacuum processing stage.
(2) The conversion of the low molecular weight pre-gel to a high molecular weight pre-gel can be controlled to within 1% of the gel point.
(3) Substantial removal of all unreacted aliphatic or aromatic polyfunctional acid or ester thereof and/or long chain aliphatic carboxylic acid or ester thereof or aromatic monocarboxylic acid or ester thereof and low molecular weight oligomers.
(4) The wiped film appratuses used are readily available, cost effective and provide a process that operates at throughputs typical of short-path distillation processes.
(5) Reduced product degradation due to lower temperatures and shorter residence times during the process compared to process that utilizes reactive extrusion alone.
(6) An optional reactive extrusion step can be incorporated or the pre-gel can be cured (crosslinked) until gel formation. However the use of the reactive wiped film vacuum processing step of the invention increases subsequent reactive extrusion throughput significantly since less conversion and little or no venting of water vapor is required in the extruder.
The present invention provides a process to produce a highly converted pre-gel, using a reactor stage to produce a low molecular weight pre-gel, and a thin film reactive vacuum processing stage to control the rate of conversion and extent of conversion of the pre-gel to create a high molecular weight pre-gel that is surprisingly close to the gel point and subsequently converting the high molecular weight pre-gel to a branched crosslinked polyester gel.
The present invention provides a process for making high molecular weight pre-gels for a crosslinked branched polyester. More specifically, the process can be used to prepare crosslinked branched aliphatic biodegradable polyesters which are particularly useful in food applications such as chewing gum bases, or the process can be used to prepare aromatic or non-biodegradable polyesters, which are useful for non-food applications including cosmetics, baking agents, customized emulsions, inks, pigment and the like.
In an embodiment, the invention provides a process for making a high molecular weight pre-gel for a cross-linked branched polyester comprising:
a) reacting polyester precursor units via condensation in a continuous or semi-continuous, or batch reactor stage to form a low molecular pre-gel; and
b) further reacting the low molecular weight pre-gel via condensation polymerization in a continuous thin-film reactive vacuum processing stage to form a high molecular weight pre-gel.
In another embodiment, the invention provides a crosslinked branched aliphatic biodegradable gum base and chewing gum composition made by process of the present invention.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.